Solar cell

This application is a divisional of U.S. application Ser. No. 16/459,789, filed Jul. 2, 2019, the content of which is hereby incorporated by reference herein.

The present disclosure relates to a solar cell.

Thin-film photovoltaic is a key technology among future low cost and sustainable renewable energy sources. Organic-inorganic (hybrid) lead halide perovskite solar cells have been proposed for photovoltaic applications because of their impressive power conversion efficiencies (PCEs), now exceeding 21%. (see Kojima et al., J. Am. Chem. Soc. 131, 6050-6051 (2009); Lee et al., Science 338, 643-647 (2012); Yang et al., Science 348, 1234-1237 (2015)). The perovskite thin-film absorber can be deposited by simple solution or sublimation methods, hence with a large potential for the preparation of inexpensive photovoltaic devices. The high PCEs are the result of the very high absorption coefficient and mobilities of the photogenerated electrons and holes of hybrid perovskites.

In order to prepare high performance solar cells, homogeneous perovskite films with a high degree of crystallinity are needed in order to reduce the trap concentration and achieve an adequate mobility of the charge carriers (see Nie et al., Science 347, 522-525 (2015)). While the use of the archetype perovskite, methylammonium lead iodide (MAPbI), can lead to high efficiency devices, a further decrease of the bandgap by incorporation of formamidinium (FA), allows for the harvesting of additional near-infrared photons (see Pellet et al., Angewandte Chemie International Edition 53, 3151-3157 (2014)). When such a mixed organic cation perovskite is further stabilized by replacing part of the iodide with bromide, the champion material for perovskite cells, (FAPbI)(MAPbBr), is obtained (see Yang et al., Science 348, 1234-1237 (2015); Jeon et al., Nature 517, 476-480 (2015); Bi et al., Science Advances 2 (2016)).

Different solar cell architectures have been used. One of them derives from dye-sensitized solar cells, and consists of a transparent conductive substrate coated with a mesoporous or planar TiOlayer (n-type, hence acting as the electron transport layer, ETL) into or onto which the perovskite light absorbing layer is applied. A hole transport layer (HTL, p-type), usually consisting of organic semiconductors is then deposited from solution on top of the perovskite and the device is finished with an evaporated top electrode (see Stranks et al., Science 342, 341-344 (2013); Eperon et al., Advanced Functional Materials 24, 151-157 (2014); Conings et al., Advanced Materials 26, 2041-2046 (2014); and Chen et al., Journal of the American Chemical Society 136, 622-625 (2014)).

Another configuration is inverted compared to the above mentioned one, and the conductive substrate is coated with a HTL, followed by the perovskite absorber and an ETL, coated with a suitable evaporated top electrode (see Wu et al., Energy & Environmental Science 8, 2725-2733 (2015); Zhou et al., Science 345, 542-546 (2014). While these two device configuration have been identified as “conventional” and “inverted”, such devices may rather be referred to as n-i-p device and p-i-n device.

Chen et al. (Science 350, 944-948 (2015)) demonstrated that the PCE in planar devices may be limited by the conductivity of the metal oxide layers, which can be increased by doping these layers. This was achieved by incorporating heteroatoms with different valences into the solution processed metal oxides, although leading only to a small increase in the conductivity (approximately one order of magnitude). Thus, only very thin metal oxide transport layers (<20 nm) could be used otherwise PCE would drop significantly.

Most reported organic-inorganic (hybrid) lead halide perovskite solar cells that are employing a vacuum deposited perovskite light absorbing layer do then employ charge transport layers processed from solution. Fully vacuum processed solar cell devices would offer the additional advantage of being compatible with temperature sensitive substrates, allowing for conformal coatings on non-planar substrates and for the straightforward implementation into tandem solar cells (see Polander et al., APL Materials 2, 081503 (2014)). Besides the high sophistication level of the deposition systems required for vacuum processing, they have been implemented in the electronic industry since long demonstrating high throughput and reliability (Ono et al., Journal of Materials Chemistry A (2016)).

By selecting certain hole transport molecules with regard to the energy levels of the conduction and valence band of the perovskite, open circuit voltages (Voc) as high as 1.1 V were demonstrated (Polander et al., APL Materials 2, 081503 (2014); Kim et al., Organic Electronics 17, 102-106 (2015), Ono et al., Journal of Materials Chemistry A (2016)). The highest efficiency (15.4%) was measured for a device with rather high hysteresis (14.0% PCE was obtained for the same cell measured in the opposite bias scan direction), which used single layers of undoped organic molecules as the charge extraction layers (see Ke et al., Journal of Materials Chemistry A 3, 23888-23894 (2015)).

EP 3 242 340 A1 discloses solar cells having a first electrode, a second electrode and a stack of layers provided between the first and the second electrode. The stack of layers comprises a light absorbing layer comprising an absorber compound provided with a perovskite crystal structure. It is further disclosed that a p-type dopant layer is provided between the first electrode and the light absorbing layer and that, at the same time, an n-type dopant layer is provided between the light absorbing layer and the second electrode.

State of the art solar cells may suffer from low open-circuit voltage, low short-circuit current, low efficiency, short lifetime and/or low fill factor.

It is, therefore, an object of the present invention to provide a solar cell overcoming drawbacks of the prior art, in particular, to provide a solar cell having improved power conversion efficiency and, at the same, improved stability and life time. It is a further object to provide a solar cell with high rectifications resulting in a high fill factor.

The above objects are achieved by a solar cell comprising a first electrode; a second electrode; and a stack of layers provided between the first electrode and the second electrode; wherein the stack of layers comprises one light absorbing layer provided with a perovskite crystal structure; and at least one dopant layer, wherein the at least one dopant layer consists of one or more n-type dopant material(s); or one or more p-type dopant material(s). In one embodiment there may be a number of the same type of dopant layer, either n-type or p-type, provided in the same stack of layers.

In context of the present invention if it is referred to “at least one dopant layer” all of the respective layers are addressed if not explicitly mentioned else.

According to the invention, the stack of layers comprises one light absorbing layer and at least one dopant layer. In this regard, it has to be understood that the stack of layers comprises exactly one light absorbing layer (and not two or more light absorbing layers) and exactly one type of dopant layer, either n-type or p-type. In other words, in case that the stack of layers which is comprised in the solar cell of claimcomprises only one dopant layer, it is provided that this dopant layer is either of the n-type or of the p-type. In case that the layer stack which is comprised in the solar cell comprises two or more dopant layers it is provided that either (first alternative) all dopant layers comprised in the single stack of layers are of the n-type, i.e. are all formed by one or more n-type dopant materials, or (second alternative) are all of the p-type, i.e. are all formed of one or more p-type dopant materials.

In one embodiment, the stack of layer comprises one light absorbing layer and one dopant layer.

In another embodiment, the stack of layers comprises one light absorbing layer and two or more dopant layers, wherein the two or more dopant layers are all either of the n-type or of the p-type, i.e. are all either formed by one or more n-type dopant materials or are all formed by one or more p-type dopant materials.

In a first alternative, the at least one dopant layer consists of one or more n-type dopant material(s). In a second alternative, the dopant layer consists of one or more p-type dopant material(s). In this regard “consisting of” means that the dopant layer exclusively contains one type of dopant material, i.e. either a single n-type dopant materials/a mixture of different n-type dopant materials or, alternatively, one single p-type dopant material/a mixture of different p-type dopant materials but not a mixture of n-type dopant materials together with p-type dopant materials or of the respective dopant materials with other materials.

In particular, it may be provided that the at least one dopant layer is free of any charge transport materials. Exemplary respective charge transport materials are disclosed herein but are not limited thereto.

In an embodiment where the layer stack which is comprised in the solar cell comprises two or more dopant layers it is provided that the two or more dopant layers are separated by layers consisting of charge transport materials. In this embodiment the dopant layer is in direct contact with the adjacent layer consisting of charge transport materials. These layers of charge transport materials are free of n-type dopants and free of p-type dopants. The use of such layers of charge transport materials may increase the power conversion efficiency of a solar cell or the stability and life time of a solar cell according to this invention.

In another embodiment it is provided that the dopant layer and the electrode are separated by a layer consisting of charge transport materials. In this embodiment the layer consisting of charge transport materials is in direct contact with the adjacent dopant layer on the one side and with the electrode on the other side. The layer of charge transport materials is free of n-type dopants and free of p-type dopants. The use of such layer of charge transport materials may increase the power conversion efficiency of a solar cell or the stability and life time of a solar cell according to this invention.

In another embodiment it is provided that the dopant layer and the light absorbing layer are separated by a layer consisting of charge transport materials. In this embodiment the layer consisting of charge transport materials is in direct contact with the adjacent dopant layer on the one side and with the light absorbing layer on the other side. The layer of charge transport materials is free of n-type dopants and free of p-type dopants. The use of such layer of charge transport materials may increase the power conversion efficiency of a solar cell or the stability and life time of a solar cell according to this invention.

Besides the one light absorbing layer and the at least one dopant layer the stack of layers may contain a variety of further layers.

According to the invention, if an n-type dopant layer is present in the (single) stack of layers, a p-type dopant layer is not present in the same stack of layers. If a p-type dopant layer is present in the stack of layers, then the n-type is not present in the same stack of layers. That is, the presence of further types of pure dopant layers in a single stack of layers-besides the at least one dopant layer—is excluded.

It was surprisingly found by the inventors that the insertion of one type of thin dopant layer between the electrode and the perovskite absorber layer increases the power conversion efficiency of a solar cell and, at the same time, the stability and life time is notably improved.

Furthermore, it was surprisingly found by the inventors that the use of only one type of dopant layer in the same layer stack as referred to herein only on one side of the light absorber layer in the same layer stack leads to diodes with high rectifications, as evidenced by a high fill factor. The generic architectures identified herein can yield very efficient and rather stable solar cells and might be used in a wide range of planer type perovskite solar cells and multi-junction architectures.

A solar cell comprises a first electrode and a second electrode and at least one stack of layers provided between the first electrode and the second electrode. The at least one stack of layers comprises a first light absorbing layer provided which may have a layer thickness of about 200 nm to about 700 nm, and comprises an absorber compound provided with a perovskite crystal structure.

In one embodiment, it may be provided that the at least one dopant layer may contain two or more dopant sub-layers wherein all of the dopant sub-layers are of the same type (either p-type or n-type) as the at least one dopant layer. That is, if the at least one dopant layer consists of one or more n-type dopant materials, all dopant sub-layers consist of n-type dopant materials which may be the same or different from each other. Likewise, in case that the at least one dopant layer consists of one or more p-type dopant materials each dopant sub-layer which may be comprised in the at least one dopant layer consists of one or more p-type dopant materials which may be selected the same or different from each other.

The inventive solar cell may contain only one stack of layers. In alternative embodiments, the solar cell may contain two or more different stacks of layers. A stack of layers in accordance with the present invention is a layer stack containing exactly one light absorbing layer and exactly one type of dopant layer, wherein the at least one dopant layer in the stack of layers consists of one or more n-type dopant material(s); or, alternatively, one or more p-type dopant material(s). In the embodiment where the solar cell comprises two or more different stacks of layers, it may be provided that all of the stacks of layers contain the same type of the at least one dopant layers (for example only n-type or only p-type) or that the stacks each individually contain a different type of the at least one dopant layer.

In case that the solar cell contains more than one stack of layers, the different stacks of layers may be separated from each other and connected with each other by interconnecting layers wherein an interconnecting layer is arranged between the first electrode and the second electrode and a first stack of layers and a second stack of layer and in direct contact with both stacks of layers. Respective interlayers and materials for forming the same are well-known from the prior art, for example WO2007/071451 A1, WO08/077615 A1 or WO2010/132236 A1. The interconnecting layer in terms of the present disclosure is not the at least one dopant layer.

In one embodiment the at least one dopant layer is arranged between the first electrode and the light absorbing layer.

In one further embodiment the at least one dopant layer is arranged between the second electrode and the light absorbing layer.

In a further embodiment the at least one dopant layer is in direct contact with the first electrode.

In one embodiment the at least one dopant layer is in direct contact with the second electrode.

In one further embodiment the at least one dopant layer is in direct contact with the light absorbing layer.

In a further embodiment the solar cell comprises two or more layer stacks and optionally at least one interconnecting layer, wherein the interconnecting layer is arranged between two of the different stacks of layers.

In one embodiment the p-type dopant material is an organic, a metal-organic or an organo-metallic compound, wherein the total amount of electron withdrawing groups in the organic, metal-organic or organo-metallic compound is from 17 atomic percent to 90 atomic percent, wherein the electron withdrawing groups are independently selected from the group consisting of fluorine, chlorine, bromine and CN.

In a further embodiment, the total number of electron withdrawing groups in the p-type dopant material which is an organic, a metal-organic or an organo-metallic compound may be equal to or higher than 4.

In one further embodiment the n-type dopant material is selected from the group consisting of metals, metal salts, metal complexes and mixtures thereof.

In one embodiment the metal is selected from the group consisting of alkali metals, alkaline earth metals, transition metals and mixtures thereof.

In one further embodiment the transition metal is selected from rare earth metals.

In a further embodiment the metal salt is selected from the group consisting of alkali metal salts, alkaline earth metal salts, rare earth metal salts and mixtures thereof.

In one further embodiment the alkali metal salt is selected from the group consisting of LIF, LiCl, LiBr, Lil and mixtures thereof, alternatively is LiF.

In one embodiment the metal complex is an organic alkali metal complex, alternatively an alkali metal complex, alternatively LiQ, an alkali borate or a mixture thereof.

In one further embodiment the thickness of the at least one dopant layer is from 0.1 to 25 nm, alternatively from 0.1 to 10 nm, alternatively from 0.1 to 5 nm, alternatively from 0.1 to 3 nm.

In a further embodiment the at least one dopant layer is a self-assembled monolayer.

Finally, the object is achieved by a solar panel comprising the solar cell as defined herein.

In the following details as to the layers and the materials thereof which may be used in accordance with the invention will be described.

p-Type Dopant

In accordance with the invention, the p-type dopant (=p-type dopant material) may be an organic compound, a metal-organic compound or an organo-metallic compound. It may be preferred that the amount of electron withdrawing groups in the organic compound (respectively the organometallic compound) is from 17 to 90 atomic percent, wherein electron withdrawing groups in this regard are preferably independently selected from the group consisting of fluorine, chlorine, bromine and CN.